U.S. patent application number 11/573051 was filed with the patent office on 2008-04-24 for light engine.
This patent application is currently assigned to KONINKLIJKE PHILIPS ELECTRONICS, N.V.. Invention is credited to Johan Marra, Hans Van Sprang.
Application Number | 20080094835 11/573051 |
Document ID | / |
Family ID | 35170024 |
Filed Date | 2008-04-24 |
United States Patent
Application |
20080094835 |
Kind Code |
A1 |
Marra; Johan ; et
al. |
April 24, 2008 |
Light Engine
Abstract
The invention describes a Light engine (1,2,3,4,5) comprising a
chamber (6) with at least one aperture (7) and a number of LED
elements (13) positioned inside this chamber, where effectively all
inner surfaces of the chamber (6) are realized as high-reflective
surfaces (20) which are essentially non-absorbing towards light
within a desired wavelength region
Inventors: |
Marra; Johan; (Eindhoven,
NL) ; Van Sprang; Hans; (Waalre, NL) |
Correspondence
Address: |
PHILIPS INTELLECTUAL PROPERTY & STANDARDS
P.O. BOX 3001
BRIARCLIFF MANOR
NY
10510
US
|
Assignee: |
KONINKLIJKE PHILIPS ELECTRONICS,
N.V.
EINDHOVEN
NL
|
Family ID: |
35170024 |
Appl. No.: |
11/573051 |
Filed: |
August 2, 2005 |
PCT Filed: |
August 2, 2005 |
PCT NO: |
PCT/IB05/52583 |
371 Date: |
February 1, 2007 |
Current U.S.
Class: |
362/247 ;
362/373 |
Current CPC
Class: |
F21V 2200/17 20150115;
Y10S 362/80 20130101; F21K 9/64 20160801; F21Y 2115/10 20160801;
F21K 9/62 20160801; F21S 41/37 20180101; G02B 6/0065 20130101; G02B
6/0073 20130101; F21S 41/24 20180101; G02B 6/0068 20130101; F21V
7/30 20180201; F21V 31/04 20130101; F21V 29/58 20150115 |
Class at
Publication: |
362/247 ;
362/373 |
International
Class: |
F21V 7/22 20060101
F21V007/22; B60Q 1/00 20060101 B60Q001/00; F21V 29/00 20060101
F21V029/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 6, 2004 |
EP |
04103783.9 |
Claims
1. Light engine (1, 2, 3, 4, 5) comprising a chamber (6) with at
least one aperture (7) and a number of LED elements (13) positioned
inside this chamber, where effectively all inner surfaces of the
chamber (6) are realized as high-reflective surfaces (20) which are
essentially non-absorbing towards light within a desired wavelength
region.
2. A light engine according to claim 1, comprising outcoupling
means (15, 15', 16, 17, 19) for enhancing outcoupling of the light
emitted by the LED elements (13) into the chamber (6).
3. A light engine according to claim 1, where the high-reflective
surfaces (20) are realized by a diffuse-reflective material (12,
18) distributed over the inside surface of the chamber walls (10,
10').
4. A light engine according to claim 3, where the
diffuse-reflective material (12) is enclosed between a transparent
covering plate (11, 11') and the inside surface of the chamber
walls (10, 10').
5. A light engine according to claim 3, where the
diffuse-reflective material (12) comprises a reflective dry powder
(12).
6. A light engine according to claim 4, where the transparent
covering plate (11) covers the LED elements (13), and the
outcoupling means (15, 15') comprise a number of transparent
outcoupling elements (15, 15') each of which extends from a light
emitting surface of an associated LED element (13) to the
transparent covering plate (11).
7. A light engine according to claim 6, where the transparent
outcoupling elements (15) have a cross section which is wider at an
interface between the outcoupling element (15) and the transparent
covering plate (11) than at an interface between the outcoupling
element (15) and the associated LED element (13).
8. A light engine according to claim 1, where the outcoupling means
(19) comprise transparent domes (19) each of which is optically
connected to a light emitting surface of an associated LED
element.
9. A light engine according to claim 8, where the domes (19)
protrude through holes (22) in a covering plate (11', 21) covering
the chamber wall (10) on which the LED elements are mounted.
10. A light engine according to claim 3, where the reflective
material (12) distributed over the inside surface of the chamber
walls (10, 10') comprises a light converting substance.
11. A light engine according to claim 1 where LED elements of
different wavelength characteristics are positioned inside the
chamber.
12. A light engine according to claim 1 comprising a light
collimating element (8) positioned at the aperture (7) of the
chamber (6).
13. A light engine according to claim 1, where the chamber (6) is
filled with a material (22) which has a refractive index that
approaches or, preferably, matches the refractive index of the
transparent covering plate (11, 11') and/or of the outcoupling
means (15, 15', 19) and/or of the collimating element (8) and/or of
the LED elements (13).
14. A light engine according to claim 13, where the material (22)
is a liquid material that is also used for the front-end cooling of
the LED elements (13).
15. A light engine according to claim 1, where an aperture fraction
(f) defined by the ratio of a surface area (A.sub.exit) of the
aperture (7) to a total interior surface area (A.sub.engine) of the
light engine ((1, 2, 3, 4, 5), which includes the surface area
(A.sub.exit) of the aperture (7), is preferably .ltoreq.0.15, more
preferably .ltoreq.0.1.
16. Automotive light system enclosing a light engine (1, 2, 3, 4,
5) according to claim 1.
Description
[0001] This invention relates to a light engine comprising a
chamber with at least one aperture and a number of LED elements
positioned inside this chamber.
[0002] A general need exists for highly intense and highly luminous
localised light sources for the purpose of, for example, coupling
this light into one or into a plurality of optical fibres, thereby
allowing the light to be transported away from a single light
source to several remote locations, e.g. to a number of automotive
headlights/taillights etc., or, for example, for emitting a
concentrated very bright light beam directly into the outside world
or into some embodiment of a light guide or into a collimating
element for the purpose of shaping and/or collimating the emitted
light beam according to certain desired specifications. An example
is a light engine comprising an intense single light source, for
example a HID lamp. With secondary optics, usually comprising at
least a parabolic mirror and a collimating lens, the light from the
light source is projected and focused onto an optical fibre array,
which allows for light transport. Examples of the latter include a
display backlight and an automotive headlight. In recent years,
because of some well-known advantages of LED light sources when
compared with traditional light sources, interest in the use of LED
light sources instead of traditional light sources has grown
considerably.
[0003] During the past decade, the technology concerning the design
and manufacture of LEDs, particularly solid-state inorganic LEDs,
has rapidly improved up to the point where inorganic white-light
emitting LEDs can now be manufactured at an efficiency of just over
40 lm/Watt. This clearly surpasses that of traditional white
incandescent lamps (16 lm/Watt at best) and most halogen lamps
(30-35 lm/Watt at best). The lumen output from a single LED die has
now increased to well beyond 100 lm and it is expected that in a
few years it should be possible to achieve an efficiency of 75
lumens/Watt at an input power of 2.7 Watt per LED die, thus
producing 200 lumen/LED. On the other hand, the limited luminous
output per LED die still presents a hurdle to be overcome on the
way to a general application of LEDs for lightning purposes in the
foreseeable future. General lighting sources must produce luminous
fluxes within the range of 500-1000 lumens for domestic use, and
1000-3000 lumens for occupational use, i.e. the current output of
ordinary incandescent and fluorescent lighting sources. This can
only be accomplished with LEDs when the light output from up to a
few dozen LED dies is combined within a single fixture, giving a
so-called light engine. In itself this should not be a problem,
however, it starts to be a problem when a high-brightness light
source is required, because, for instance, the emitted light from
all LEDs combined has to be collimated with the aid of a
small-sized compact collimating element. A well-known example of
the latter is an automotive headlamp. Here, one commonly makes use
of H7 halogen lamps (55 W input power) which emit some 1500 lumens.
These lumens are emitted at a brightness of about 30 Mcd/m.sup.2.
In case of using a Xenon HID lamp, the achieved brightness
increases to about 80 Mcd/m.sup.2. In contrast, when a single 1
mm.sup.2 LED die is made to emit 50 lumens of white light, which is
about the best that can be achieved with the technology currently
available, the brightness of a single die is only 8 Mcd/m.sup.2,
still a few times below that of a halogen lamp and an order of
magnitude below the brightness of a conventional HID lamp. The
situation significantly worsens when multiple LED dies are
required, due to the necessary spacing between adjacent dies.
[0004] One example of a LED light engine (also called `light
generator`), capable of combining the light output from various LED
dies together to achieve a single concentrated (collimated) beam of
emitted light, is disclosed in U.S. Pat. No. 6,402,347. Therein,
individual LED elements are mounted on a back plate, each of them
equipped with a collimating dome. An adjacent aligned Fresnel lens
allows for the projection of the individual LED light beams onto a
single output element, for example an optical light guide. A main
problem of this system is the large light loss, which may amount up
to about 60% due to reflections from the various optical
interfaces. Other disadvantages of this light engine are its bulk,
and the required precision alignment of the secondary optics,
raising the cost for such a light engine. As yet, the size and cost
of such a LED light engine far exceeds that of an ordinary
high-intensity light source.
[0005] Therefore, an object of the present invention is to provide
a LED-based light engine which is easier and cheaper to produce,
which is of compact dimensions, and which shows a better
performance than known LED-based light engines.
[0006] To this end, the present invention provides a light engine
comprising a chamber with at least one aperture and a number of LED
elements positioned inside this chamber, where, effectively, all
inside surfaces of the chamber are realized as high-reflective,
preferably diffuse-reflective (also called `white-reflective`),
surfaces which are essentially non-absorbing towards light within a
desired wavelength region, particularly the visible region, the UV
region, and/or the infrared region. The term `high-reflective` is
to be understood as a reflectivity approaching 100%, preferably
.gtoreq.95%, more preferably .gtoreq.98%.
[0007] If, according to the invention, all inside surfaces which
are not occupied by a LED element--including the surface area
between the LED elements--are highly reflective, then essentially
all light emitted by the LED elements will leave the chamber
through the aperture, perhaps after multiple, possibly numerous,
reflections. Such a phenomenon of multiple reflection of the light
inside the chamber is known as `internal light recycling`. In such
a construction, every internal chamber surface is effectively an
emissive surface, whether it emits light itself, as is the case
with the surface of a LED element, or reflects light. The light
engine according to the invention does not comprise the internal
secondary optics from prior art light engines, and is therefore
more economical to manufacture. If desirable, the light engine
according to the invention can be provided with external secondary
optics, preferably provided near the light engine aperture, for the
purpose of shaping and/or collimating the emitted light beam from
the light engine.
[0008] In a chamber designed in this way, constructed as a
`integrating sphere` or so-called `Ulbricht sphere`, by far the
greatest part of the light emitted by the LEDs within the chamber
will indeed exit the chamber through the aperture. Evidently, the
efficiency of the entire light engine ultimately depends on the
attained reflectivity of the inside surfaces of the chamber.
Although the reflectivity of the inside surfaces is unlikely to
reach exactly 100%, this limit can still be reasonably well
approached. That an exceptionally good performance can be attained
with a light engine according to the present invention is
demonstrated in the following, whereby the values ultimately
attained will always depend on the exact construction parameters of
the light engine, such as the LED packing density within the
chamber, the reflectivity of the LEDs, and the size of the aperture
opening in relation to the total inner surface area of the light
engine that is exposed to light. The exact construction parameters
should therefore be chosen to suit the desired application.
[0009] In the following, it is assumed that the LEDs are
solid-state inorganic LED dies, since these are currently available
with sufficient luminous intensity. Nevertheless, any other
electro-luminescent elements can be used, for example, laser
diodes, other types of semiconductor light emitting elements or
organic LEDs, as long as these deliver sufficient performance.
Therefore, the term `LED` in the following is to be regarded as a
synonym for any type of appropriate electro-luminescent
element.
[0010] The dependent claims and the subsequent description disclose
particularly advantageous embodiments and features of the
invention.
[0011] The reflectivity of the inside surfaces can be achieved in
basically any manner. It is only critical that the reflectivity be
sufficiently high, preferably .gtoreq.98%. Preferably, the
highly-reflective surfaces may be realized by distributing a
diffuse reflective material over the inside surface of the chamber
walls. For example, the inside surfaces can be coated with an
appropriate material in the form of a particle/binder coating of
sufficient thickness.
[0012] In a particularly preferred embodiment of the invention, the
diffuse-reflective material is enclosed between the inside surface
of the chamber walls, and a covering plate which is transparent in
at least the desired wavelength region. The diffuse reflective
material is thus `sandwiched` between the inside surfaces of the
chamber walls and the transparent covering plate. This construction
permits use of a diffuse reflective material such as a reflective
dry powder, preferably a free-flowing powder. Suitable reflective
white powder may comprise inorganic particles such as
Al.sub.2O.sub.3, YBO.sub.3, BaSO.sub.4, TiO.sub.2,
Ca-pyrophosphate, Ca-halophosphate, MgO, or mixtures of these
particles. The absence of any organic binder material increases the
reflectivity of the powder particles and avoids gradual
discolouration over time. Use of Ca-pyrophosphate at an average
particle diameter of 5-15 .mu.m is particularly recommended because
of its cheapness and ready availability, chemical purity,
resistance to high temperatures (>1000.degree. C.), its ability
to behave as a free-flowing powder when mixed with approximately 1%
w/w Alon-C nanoparticles (i.e. Al.sub.20.sub.3 nanoparticles,
Degussa GmbH, Germany) which is useful for enabling an easy filling
of the relatively narrow space between the inside surfaces of the
chamber and the covering plate with dry powder particles, and its
proven non-absorbing characteristics towards visible light after
annealing at 900.degree. C. With Ca-pyrophosphate powder, the
reflective powder layers should preferably have a thickness of at
least 2 mm in order to accomplish a reflectivity of at least
98-99%.
[0013] In order to attain a greatest possible efficiency as regards
converting the input power into light, it is necessary that as much
as possible of the light generated inside the LED elements actually
exits the LEDs into the chamber interior. This is not without
problems, owing to the internal reflections arising at the boundary
layer between the LED die surface and the surroundings. Therefore,
in a preferred embodiment of the invention, the light engine
comprises outcoupling means for enhancing outcoupling of the light
emitted by the LED elements into the chamber.
[0014] The outcoupling means might comprise transparent domes,
made, for example, from a silicone resin and/or some organic
polymeric material, each of which is optically connected to a light
emitting surface of an associated LED element. Preferably, the
domes protrude through holes in a covering plate, when such a
covering plate is used for covering the chamber wall on which the
LED elements and/or LED device bodies are mounted, and for
covering/sandwiching a diffuse-reflective white material. The
presence of such transparent domes around the LED elements promotes
light outcoupling from the LED dies. On the other hand, their
presence may adversely affect the efficiency with which the
internal light recycling inside the light engine chamber can be
realised. Depending on the LED characteristics and the wavelengths
involved, light absorption might take place at the bottom of the
domes inside or directly adjacent to the associated LED elements.
Furthermore, a highly-reflective coating on the inside of the light
engine is present only on the inside wall surface areas located in
between the domes, the domes having a significantly larger
cross-section than the cross-section of the associated LED dies
themselves.
[0015] Therefore, in an alternative embodiment, the LED domes are
simply omitted. The white-reflective coatings can then be applied
in between the LED die elements and will cover a substantially
larger fraction of the inside wall surface area when compared with
the situation in which domes are used. As light outcoupling from
bare LED dies is intrinsically less efficient, the exposed LED die
surfaces are preferably covered with a transparent scattering
coating layer that is in optical contact with the die, or a
microstructure is applied directly to the LED die surfaces. These
latter measures promote light outcoupling from the LED dies.
[0016] In a most preferred embodiment, the transparent covering
plate which covers the reflective material also covers the LED
elements, and the outcoupling means comprises a number of
transparent outcoupling elements, each of which extends from a
light emitting surface of an associated LED element to the
transparent covering plate. Thereby, the optical transparent
outcoupling elements may itself form part of the covering
plate.
[0017] The transparent outcoupling elements preferably have a cross
section which is wider at an interface between the outcoupling
element and the transparent covering plate than at an interface
between the outcoupling element and the associated LED element. For
example, the transparent outcoupling elements may have a conical,
parabolic, or pyramidal form featuring a cross-section that widens
in the direction facing away from the associated LED elements. Such
a form ensures that the transparent outcoupling element not only
helps to out-couple the light generated in the LED die and to
conduct it, like a light conductor, through the transparent
covering plate into the inside of the chamber, but also helps to
act as a collimator for the LED, limiting the emission angle of the
LED.
[0018] Essentially, any LED element can be used, for example LED
dies that are coated with a light converting substance (usually
called fluorescent or just `phosphor` coating). The phosphor
coating of such LEDs ensures that at least a part of the light
emitted by the LED at a certain wavelength is converted into a
different wavelength, so that, overall, light is emitted with a
desired wavelength characteristic, i.e. a certain colour. An
optical interference layer may be arranged between the LED die and
the phosphor coating on the LED die surface serving to promote the
transmission of the light generated inside the LED die into the
phosphor layer and to diminish the transmission of
phosphor-converted light from the phosphor layer into the LED
die.
[0019] In case phosphor conversion LEDs are used, the light
converting substance--such as phosphor particles--can be
distributed either on or in the reflective material, for example
the white reflective powder, which is distributed either as a
particle/binder coating over the inside surfaces of the chamber
walls or which is sandwiched as a binder-free dry powder layer
between the inside surfaces of the chamber walls and a transparent
covering plate. This is not only easier and cheaper from a
processing/packing point of view, but also offers a strategy which
counteracts phosphor saturation phenomena and should help to raise
the lumen output from the dies. Incorporating the phosphors in the
diffuse reflective white powder through e.g. a simple mixing of the
dry powders, simplifies the manufacture of the LED elements and
avoids phosphor saturation at high light intensities since a larger
amount of phosphor can then be spread across a relatively large
surface area. The amount and positioning of the phosphors in the
diffuse-reflective white powder layer or the diffuse-reflective
particle/binder coating can be optimized such that a suitable
colour point is obtained. In this case, phosphor-free LED elements
can also be used.
[0020] Additionally or alternatively, LED elements of differing
wavelength characteristics, e.g. red, green, blue can be used,
positioned as desired within the light engine. The issue of
adequate colour mixing is automatically solved when a light engine
according to the invention is utilised, since the individual LED
dies cannot directly be observed from the outside and internal
colour mixing is taken care of by the internal light recycling
process.
[0021] The aperture can consist basically of a simple opening in
the chamber wall. The effect of the aperture parameters on the
performance of the light engine will be discussed in detail later.
A light conductor element can be arranged near, in, or on the
aperture, for example an optical fibre or similar, in which the
light, generated in the light engine, is caught. In a preferred
embodiment of the invention, a beam-forming element is arranged in
or near the aperture. A light collimating element, e.g. in the form
of a lens, a conical element, a pyramidal element, or a parabolic
element, is particularly preferred. The light exiting through the
aperture is collimated within a defined emission angle and/or is
shaped within a defined spatial/angular light intensity
distribution pattern by such a collimating element.
[0022] To minimise light losses through internal reflections at the
various optical interfaces existing within the light engine, for
instance the interfaces between the chamber interior and the
transparent covering plate and/or between the chamber interior and
the outcoupling means and/or between the chamber interior and the
collimating element disposed at the light engine aperture, and/or
between the chamber interior and the LED die surfaces, the chamber
is preferably filled with a material which has a refractive index
that approaches or, more preferably, matches the refractive index
of the transparent covering plate and/or of the outcoupling means
and/or of the collimating element and/or of the LED elements, and
which therefore reduces or even eliminates the `optically
visibility` of the various optical interfaces with respect to
visible light and/or with respect to the light generated inside and
emitted from the LED dies.
[0023] This material may be an organic medium such as a transparent
liquid, particularly an oil, or a solid resin, particularly a
silicone resin, possessing the desired (matching) refractive index,
and which is preferably substantially non-absorbing with respect to
visible light and/or with respect to the light generated inside and
emitted from the LED dies. This measure also minimises Fresnel
reflections from the optical element positioned at the aperture of
the light engine when the filling material inside the cavity is in
optical contact with the said optical element. A preferred
embodiment is obtained when the material is a liquid material that
is also used for the front-end cooling of the LED elements.
Preferably, the liquid material is then pumped around as a fluid
between the light engine cavity and some additional external
cooling device in order to increase the cooling effect of the
fluid.
[0024] The light engine according to the invention can be used for
any LED luminaire application, characterised in that the luminaire
possesses a light output aperture of restricted-area, with a light
output beam of adjustable brightness and colour, from where the
light can be transported to several remote locations, particularly
in automotive light systems such as automotive headlamps. The
brightness of the emitted light output beam can be conveniently
adjusted by altering the electrical power delivered to individual
LED elements inside the light engine. The colour of the emitted
light output beam can also be adjusted by altering the electrical
power delivered to individual LED elements under the circumstance
that LED elements of differing wavelength characteristics are
present inside the light engine, e.g. red, green, and blue LED
elements.
[0025] Other objects and features of the present invention will
become apparent from the following detailed descriptions considered
in conjunction with the accompanying drawings. It is to be
understood, however, that the drawings are designed solely for the
purposes of illustration and not as a definition of the limits of
the invention. In the drawings, wherein like reference characters
denote the same elements throughout:
[0026] FIG. 1 shows a first embodiment of a light engine according
to the invention;
[0027] FIG. 2 shows an enlarged image of a part of the walls of the
chamber of the light engine according to FIG. 1;
[0028] FIG. 3 shows an enlarged portion of a wall of a chamber of a
light engine according to a second embodiment of the invention;
[0029] FIG. 4 shows an enlarged portion of a wall of a chamber of a
light engine according to a third embodiment of the invention;
[0030] FIG. 5 shows an enlarged portion of a wall of a chamber of a
light engine according to a fourth embodiment of the invention;
[0031] FIG. 6 shows a fifth embodiment of a light engine according
to the invention;
[0032] FIG. 7 shows a sixth embodiment of a light engine according
to the invention;
[0033] FIG. 8 shows a seventh embodiment of a light engine
according to the invention;
[0034] FIG. 9 shows an eight embodiment of a light engine according
to the invention;
[0035] FIG. 10 shows a simplified schematic representation of the
shape of a chamber for a light engine according to a ninth
embodiment of the invention.
[0036] FIG. 11 is a diagram illustrating the influence of the
aperture fraction f on: [0037] the fraction T of the internally
generated light that is emitted (transmitted) from the light
engine; [0038] the brightness ratio B denoting the brightness of
the emitted light beam from the aperture of the light engine
normalised with respect to the brightness of an individual LED
element; [0039] a quality parameter Q.
[0040] FIG. 12 is a diagram showing the dependence of the quality
parameter Q on the aperture fraction f for various reflectivities
of the inside reflecting wall surfaces and LED surfaces.
[0041] FIG. 13 is a diagram showing the dependence of the quality
parameter Q on the aperture fraction f for several packing
densities .theta..sub.LED of the LED elements on the inside walls
of the light engine.
[0042] FIG. 14 is a diagram showing the dependence of the light
concentration factor L on the collimation angle .theta..sub.C for
various aperture fractions f.
[0043] FIG. 15 is a diagram showing the dependence of a light
concentration factor L on the collimation angle .theta..sub.C for
various LED packing densities .theta..sub.LED for a particular
first aperture fraction f.
[0044] FIG. 16 is a diagram showing the dependence of a light
concentration factor L on the collimation angle .theta..sub.C for
various LED packing densities .theta..sub.LED for a particular
second aperture fraction f.
[0045] The dimensions of the objects in the figures have been
chosen for the sake of clarity and do not necessarily reflect the
actual relative dimensions.
[0046] FIGS. 1 and 2 show a particularly preferred embodiment of a
light engine according to the present invention, whereby FIG. 1
shows a cross-section through the entire light engine, and FIG. 2
shows an enlarged cross-section through the chamber wall.
[0047] The light engine 1 comprises a chamber 6, constructed, for
example, in a rectangular or cylindrical manner. An opening or
aperture 7 of surface area A.sub.exit is located at the top of the
chamber 6 and connects to a collimating element 8. LED elements 13
are positioned on the inside wall 10 of the chamber 6 at a certain
distance from each other, i.e. in a particular grid, along the
mantle and on the inside surface opposite the aperture 7. These LED
elements 13 are connected via outcoupling elements 15 to a
transparent covering plate 11.
[0048] This transparent covering plate 11 is positioned in the
chamber 6 at a certain distance to the inside wall of the chamber
6. All walls 10 of the chamber 6, including the top side with the
aperture 7, are covered by the transparent covering plate 11. The
gap between the transparent covering plate 11 and the inside
surfaces of the walls 10 of the chamber 6 is filled with a diffuse
reflective white powder. Suitable candidates for the reflective
white powder are Al.sub.20.sub.3, TiO.sub.2, YBO.sub.3, BaSO.sub.4,
Ca-pyrophosphate, Ca-halophosphate, or MgO. Suitable materials for
the transparent covering plate 11 include PMMA
(polymethyl-methacrylate), PC (polycarbonate), resinous silicone
compounds, and glass. This construction ensures that all inside
surfaces 20 of the chamber 6, not occupied by a LED die, are highly
reflective.
[0049] The construction of the walls can be seen in detail in FIG.
2. Here, the individual LED dies 13 are mounted on mounting slugs
14 which, preferably, also feature a reflective top surface around
the LED dies. Transparent truncated inverted pyramids or cones
serve as outcoupling elements 15 that are optically coupled to the
transparent covering plate 11. Furthermore, these outcoupling
elements 15 are optically coupled to the LED dies 13 by means of a
resin or some other suitable glue-like material. Instead of
optically coupling these outcoupling elements 15 with resin or a
similar material to the transparent covering plate 11, they can
preferably also be directly formed as part of the transparent
covering plate 11. The outcoupling elements 15 guide the emitted
light towards the interior 9 of the light engine 1. The
cross-section of the conical outcoupling elements 15 widens in the
direction facing away from the associated LED dies 13. Preferably,
the outcoupling elements feature an angle of inclination between
5.degree. and 65.degree. with respect to the vertical, more
preferably featuring an angle of inclination between 20.degree. and
50.degree. with respect to the vertical, and most preferably
featuring an angle of inclination of about 45.degree. with respect
to the vertical.
[0050] The distance between the transparent covering plate 11 and
the inside surface of the non-transparent outer wall 10 of the
chamber 6, i.e. the thickness of the diffuse reflective powder
layer 12, is preferably about 2-3 mm. The powder layer 12 provides
the highly reflective surfaces 20 of the chamber 6, which enable
internal light recycling. A collimating element 8 is arranged on
the aperture 7, and is made from, for instance, transparent plastic
material, and receives light that is emitted from the aperture 7 of
the light engine 1. The shape of the collimating element 8 is
chosen such that substantially no light is emitted from the exit
surface of the collimating element 8 at an angle greater than the
collimation half angle .theta..sub.C measured with respect to the
propagation direction of the emitted light beam.
[0051] In order to improve the light outcoupling from the
transparent covering plate 11 into the inside of the chamber 6, and
to simplify the coupling of the light from the chamber 6 into the
collimating element 8, the interior 9 of the entire chamber 6 is
filled with a solid or liquid medium 22 which has a refractive
index approaching or, more preferably, matching that of the
transparent covering plate 11 and possibly also that of the
collimating element 8. Unwanted light-loss inducing reflections at
the boundary interfaces between the covering plate 11 and the
medium 22, and at the interface between the collimating element 8
and the medium 22 are thereby avoided or at least diminished. In
case the medium 22 is a liquid medium, the liquid can also be
utilised for front-end LED cooling purposes, for instance by
pumping the liquid medium 22 between the chamber 9 and an external
cooling device.
[0052] FIG. 3 shows a somewhat modified construction of the inside
surface of the wall 10 of the chamber 6. Here, the LED dies 13 are
mounted directly on the inside surface of the chamber wall 10. An
optical contact layer 16 is positioned on each LED die 13. This
contact layer 16 may contain scattering particles to promote light
outcoupling from the LED die 13. The transparent covering plate 11
features block-shaped outcoupling elements 15', which protrude from
the transparent covering plate 11 towards the LED die 13, acting as
an extension or bridge, and providing optical contact with the
contact layer 16. The space between the transparent covering plate
11 and the inside surface of the wall 10 is here also filled with a
reflective dry white powder 12.
[0053] FIG. 4 shows a further possible construction. As in FIG. 3,
the LED dies 13 are positioned on the inside wall 10. To facilitate
outcoupling of the light emitted by the LED die 13 through the LED
die surface, the LED dies 13 are preferably surrounded by a
transparent scattering layer 17 that is in optical contact with the
LED die surface, thereby promoting light outcoupling from the LED
dies 13 into the chamber 6. A highly diffuse-reflective white
particle/binder layer 18 covers the surfaces of the inside wall 10
that are located between the individual LEDs 13.
[0054] In FIG. 5, a further possible construction can be seen,
where LED device bodies 23, each with a LED die element (not shown
in the diagram), are mounted on the inside surface of the outer
wall 10. The LED die elements themselves are enclosed in LED domes
19, which ensure good outcoupling of the light emitted from the LED
dies. A covering plate 21, with suitable openings in a grid pattern
through which the LED domes 19 protrude, covers the LED device
bodies 23. The surface of the covering plate 21 between the LED
domes 19 is covered with a white diffuse-reflective particle/binder
coating 18 possessing a sufficient thickness to yield a highly
reflective coating layer 18.
[0055] A further example construction of a light engine 2 is shown
in FIG. 6. The basic difference between it and the example shown in
FIG. 1 is that the chamber 6 is constructed differently than that
of the light engine 1 in FIG. 1. Here, the chamber 6 features a
floor wall 10, upon which the individual LEDs are mounted as in the
example shown in FIG. 1. However, the side walls 10' now extend
conically from the floor wall 10 towards the aperture 7. No LEDs
are positioned on these side walls 10'. To give the desired highly
reflective inside surface 20, a transparent covering plate 11 is,
as for the floor wall 10, arranged at a distance of about 2-3 mm
from the inside of the side walls 10', and the space between the
covering plate 11 and the side walls 10', as well as the space
between the floor wall 10 and the covering plate 11 in between the
LED mounting elements 14, dies 13, and outcoupling elements 15 are
filled with a highly reflective white powder 12. Again, a
collimating element 8 is arranged at the aperture 7. The advantage
of this light engine 2 over the light engine 1 lies in its reduced
volume and, in particular, in its reduced height. On the other
hand, the number of LED elements relative to the total area of the
chamber's inside walls is lower, since the side walls 10' are not
occupied by LED elements.
[0056] A further embodiment of a light engine 3 according to the
present invention is shown in FIG. 7. The housing 6 of this light
engine 3 features the same geometry as the housing of the light
engine 2. However, the LED elements are mounted on the base 10 in
the same manner shown in FIG. 5, i.e. LED device bodies 23,
supporting LED domes 19 in which the LED dies (not shown in the
diagram) are enclosed, occupy the base 10. Both the surface of the
base wall 10 upon which the LEDs are mounted as well as the side
walls and the tops of the LED device bodies 23 are covered with a
white diffuse reflective coating 18 leaving only the protruding
domes 19 to remain uncoated. A transparent covering plate 11', with
suitable openings in a grid pattern through which the LED domes 19
protrude, covers the LED device bodies 23. The space between this
transparent covering plate 11' and the inside surface of the outer
wall 10 is filled with a reflective white dry powder 12. The
conical side wall 10' narrowing to the aperture 7 with the
reflective material 12 disposed between the inside surface of the
side wall 10' and a transparent covering plate 11 is constructed in
the same manner as for the light engine 2 of FIG. 6.
[0057] FIG. 8 shows a further embodiment of a light engine 4
according to the present invention, which, as regards outer housing
6, is constructed in a similar manner as the example described in
FIG. 7. Other than in the example of FIG. 7, however, neither a
transparent covering plate 11' nor a reflective white dry powder 12
are used here. Instead, the conical chamber walls 10' are now also
covered on the inside with a white diffuse-reflective
particle/binder layer 18 to give a highly reflective surface 20. In
addition, a white diffuse-reflective particle/binder layer 18 is
present on the inside surfaces of the chamber wall 10, and on the
surfaces of the LED device bodies 23 located between the
transparent domes 19.
[0058] FIG. 9 shows a further embodiment of a light engine 5
according to the present invention, which essentially only differs
from the examples in FIG. 1 and FIG. 6 in the outer shape of the
chamber 6. The lower part of the chamber 6 is cylindrically or
rectangularly shaped, with a base wall 10 and a side wall 10, each
occupied by LED elements 13 arranged in a certain grid pattern. The
upper part of the chamber 6 narrows conically to the aperture 7, in
the same way as the conically formed side wall 10' of the light
engine 2 in FIG. 6. This conical wall 10' of the upper part of the
chamber 6 is not occupied on the inside by LED elements 13, having
only a highly reflective surface 20. This highly reflective surface
20 is formed again by a transparent covering plate 11 arranged at a
distance from the walls 10, 10' and a white reflective powder 12
filling the space between the inside surface of the walls 10, 10'
and the covering plate 11.
[0059] In all cases shown in the FIGS. 6 to 9, the interior 9 of
the chamber 6 is preferably filled with a solid or liquid medium 22
possessing a suitable refractive index, as described in connection
with the light engine 1 of FIG. 1.
[0060] The different examples show that the chamber 6 can basically
have any kind of external geometry. Furthermore, it must be
stressed that the aperture 7 does not necessarily have to be a
circular opening in a side wall and that it is not necessarily
provided with an optical element 8. Any side wall, preferably of
relatively small dimensions, can be simply left out of the
construction, giving an aperture 7. This is shown by the
cylindrical chamber 6 of the simplified schematic in FIG. 10.
Basically, such a chamber 6 can have any basic surface geometry,
for example an aperture on opposite sides. For example, one can
also imagine elongated light engine cubes with both small faces
open to the outside world. This depends on the intended function of
the light engine, and the spatial constraints under which the light
engine will operate.
[0061] The exact construction parameters such as chamber geometry,
number of LED elements in the chamber, size of aperture etc.,
depend on constraints such as the maximum size of the light engine,
and the desired output parameters. The following therefore
describes how the attainable output parameters depend on the
construction parameters of the light engine:
[0062] Consider the light engine box depicted in FIG. 1 possessing
a single aperture or exit port 7 of surface area A.sub.exit, and a
total interior surface area A.sub.engine, which includes the exit
port surface area A.sub.exit. Suppose that a total number N.sub.LED
Of individual LED die elements 13, each possessing a projected)
flat top area A.sub.LED, are mounted on the inner surface of the
wall 10 of the light engine 1. Each LED element 13 is assumed to
possess a reflectivity R.sub.LED and emit a lumen flux
.phi..sub.LED from its die area A.sub.LED. A white
diffuse-reflective wall 20 of reflectivity R.sub.wall is laterally
present around the LED elements 13.
[0063] The transmitted fraction T of the internally produced light
that escapes via the aperture exit port 7 into the outside world
follows from the series:
T = A exit A engine + ( 1 - A exit A engine ) R av A exit A engine
+ ( 1 - A exit A engine ) 2 R av 2 A exit A engine +
##EQU00001##
[0064] which, with the `aperture fraction`
f = A exit A engine ##EQU00002##
[0065] is equivalent to
T = f 1 - R av ( 1 - f ) ( 1 ) ##EQU00003##
[0066] Thereby, R.sub.av denotes the averaged internal reflectivity
R.sub.av of the non-exit part of the light engine's inner wall
surface according to
R av = N LED A LED R LED + ( A engine - A exit - N LED A LED ) R
wall A engine - A exit = .theta. LED R LED + ( 1 - .theta. LED ) R
wall wherein ( 2 ) .theta. LED = N LED A LED A engine - A exit ( 3
) ##EQU00004##
[0067] denotes the fraction of the internally reflecting light
engine surface area A.sub.engine-A.sub.exit that is covered with
LED elements 13.
[0068] The above equations do not assume any specific shape of the
internal light engine wall. On the other hand, the series expansion
in Equation (1) only holds for small aperture fractions f. In the
extreme case of a light engine comprising a single flat
light-emitting surface, one has a maximum f=0.5 and, by definition,
T=1 since no reflecting surfaces are in the way of the emitting
light sources. In this case, Equation (1) erroneously predicts a
T<1 but the error is still not substantial as long as
R.sub.av>0.90, which can readily be accomplished.
[0069] For realistic light engines, an upper limit
f.apprxeq.0.3-0.4 should preferably be maintained, but the concept
of a light engine according to the invention is obviously more
interesting for much smaller values of f. For example a light
engine embodied as a square cube that is open on only one of its
six sides possesses an aperture fraction f=0.17. Smaller values for
the aperture fraction f can be easily obtained by making the cube
more rectangular while keeping only one of its two small sides
open. In a preferred embodiment of the invention, the aperture
fraction f should be .ltoreq.0.15, more preferably .ltoreq.0.1. For
example, a light engine 1 according to FIG. 1 with a diameter of 2
cm, a chamber length of 3 cm and an aperture diameter of 1 cm has
an aperture fraction f=0.03.
[0070] In case R.sub.LED=R.sub.wall=R.sub.av=1, no light losses are
present and one obtains T=1 according to Equation (1) for any
arbitrarily small aperture fraction f. This would theoretically
allow the creation of extremely high brightness levels when
f.fwdarw.0. In reality, however, this is impossible since optical
light losses can never be fully avoided.
[0071] It is therefore also of interest to derive an equation for
the obtainable brightness at the aperture exit port 7 of the light
engine 1 as a function of the system parameters. The brightness
ratio B, denoting the brightness B.sub.exit at the aperture exit
port 7 (assuming that no collimating element 8 is present)
normalised with respect to the brightness level B.sub.LED of an
individual LED die element 13, follows from
B = B exit B LED = TN LED .PHI. LED A exit .PHI. LED A LED = TN LED
A LED A exit = T N LED A LED A engine - A exit A engine - A exit A
exit = T .theta. LED ( 1 f - 1 ) = f 1 - R av ( 1 - f ) .theta. LED
1 - f f = 1 - f 1 - R av ( 1 - f ) .theta. LED = ( 1 - f ) .theta.
LED 1 - ( 1 - f ) [ .theta. LED R LED + ( 1 - .theta. LED ) R wall
] ( 3 ) ##EQU00005##
[0072] which is valid when both the LED dies 13 and the aperture
exit port 7 emit non-collimated light (i.e. Lambertian light with
.theta..sub.c=90.degree.).
[0073] In addition, it is instructive to derive a relation for a
second brightness ratio L(.theta..sub.c), also called `brightness
concentration factor` in the following, with
L ( .theta. c ) = B exit ( .theta. c ) B screen ( .theta. c ) ( 4 )
##EQU00006##
[0074] which denotes the ratio of the brightness
B.sub.exit(.theta..sub.c) of the light-emitting exit surface (which
may be the projected light-emitting exit surface A.sub.col of the
collimating element 8) to the screen-averaged brightness
B.sub.screen(.theta..sub.c) of an imaginary flat screen of surface
area A.sub.screen=A.sub.engine-A.sub.exit whereupon LED elements 13
are mounted at a packing density .theta..sub.LED. Here, light is
assumed to be emitted as a collimated beam that is angularly
bounded within a collimation half-angle .theta..sub.c with respect
to the propagation direction of the beam. For non-collimated light,
one has .theta..sub.c=90.degree..
[0075] Knowledge of the brightness concentration factor
L(.theta..sub.c) indicates whether or not a net light concentration
has been achieved by packing N.sub.LED dies together inside a light
engine 1 at a surface packing density .theta..sub.LED in comparison
with the simpler situation wherein the N.sub.LED dies are simply
mounted on a flat light emitting screen at the same surface packing
density. A value L(.theta..sub.c)>1 indicates a relative light
(brightness) concentration and a value L(.theta..sub.c)<1
indicates a relative light (brightness) dilution. Evidently, a
value of L(.theta..sub.c) as large as is practically possible, and
certainly higher than 1, is generally desirable.
[0076] In case the light engine 1 is made to emit 2D-collimated
light, as shown in FIG. 1, the relevant exit port surface area
becomes that of the projected output surface A.sub.col of the
collimating element 8 mounted on the exit port 7 of the light
engine 1. Following the etendue law, for a given collimation
half-angle .theta..sub.c, the minimum required output surface area
A.sub.col of the collimating element 8 relates to the output area
A.sub.exit of the aperture 7 of the light engine 1 in FIG. 1
according to
A col = A exit sin 2 .theta. c ( 5 ) ##EQU00007##
[0077] and thus indicates an inevitable enlargement of the emitting
surface A.sub.col at decreasing .theta..sub.c.
[0078] The screen-averaged brightness level
B.sub.screen(.theta..sub.c) relates to B.sub.LED(.theta..sub.c)
according to
B.sub.screen(.theta..sub.c)=.theta..sub.LEDB.sub.LED(.theta..sub.c)
(6)
[0079] In the embodiment of the light engine 1 according to FIG. 1,
the individual LED elements are provided with a collimating element
in the form of pyramidal outcoupling elements 15. Therefore, the
apparent light emitting surface area of an individual LED also
increases but these can be directly accommodated on the mounting
screen (the imaginary flat screen of surface area A.sub.screen
defined above for derivation of equation (4)) without enlarging the
screen as long as the LED packing density constraint
.theta..sub.LED.ltoreq.sin.sup.2(.theta..sub.c) (7)
[0080] on the flat mounting screen is satisfied. The screen surface
area A.sub.screen can then be taken to be independent of
.theta..sub.c.
[0081] From the above, and bearing in mind that B.sub.exit in
Equation (4) denotes the brightness B.sub.col at the light output
surface of the collimating element 8 mounted on the aperture
opening 7 of the light engine 1 as soon as
.theta..sub.c<90.degree., it follows that L(.theta..sub.c) can
be obtained from:
L ( .theta. c ) = B col B screen = TN LED .PHI. LED A col .theta.
LED B LED = sin 2 .theta. c .theta. LED ( TN LED .PHI. LED A exit
.PHI. LED A LED ) = sin 2 .theta. c .theta. LED B = ( 1 - f ) sin 2
( .theta. c ) 1 - ( 1 - f ) [ .theta. LED R LED + ( 1 - .theta. LED
) R wall ] ( 8 ) ##EQU00008##
[0082] For the special case .theta..sub.c=90.degree. (Lambertian
light) FIG. 11 shows calculated values for the transmission T and
the brightness ratio B as a function of the aperture fraction f at
a packing density .theta..sub.LED=0.05 and at reflectivities
R.sub.wall=0.98 and R.sub.LED=0.50 which corresponds to realistic
conditions.
[0083] It is clear from FIG. 11 that, at decreasing aperture
fraction f, the attainable brightness ratio B at the exit port
increases, albeit at a significantly decreasing lumen output, which
is proportional to the transmitted fraction T of the internally
produced light that leaves the light engine. Because lumen flux and
brightness exhibit opposite trends in their relationship with the
aperture fraction f, it makes sense to define a quality parameter Q
according to
Q = BT = .theta. LED ( 1 - f ) f [ 1 - [ .theta. LED R LED + ( 1 -
.theta. LED ) R wall ] ( 1 - f ) ] 2 ( 9 ) ##EQU00009##
[0084] In FIG. 11 the quality parameter Q is also plotted as a
function of the aperture fraction f. From this plot it becomes
apparent that Q goes through a maximum at an optimum aperture
fraction f.sub.opt. Values for f.sub.opt follow from
f opt = 1 - R av 2 - R av ( 10 ) ##EQU00010##
[0085] with R.sub.av given by Equation (2).
[0086] However, Q does not strongly depend on the aperture fraction
f near f=f.sub.opt. In case a high T is more important than a high
B (for example when a general lighting application is concerned),
one is advised to choose a value f>f.sub.opt. The reverse is
true when a high B is more important than a high T.
[0087] FIG. 12 shows calculations of the quality parameter Q factor
as a function of the aperture fraction f for various values of
R.sub.LED and R.sub.wall at a packing density .theta..sub.LED=0.05
and for .theta..sub.c=90.degree.. (I: R.sub.wall=0.98 and
R.sub.LED=0.7; II: R.sub.wall=0.98 and R.sub.LED=0.5; III:
R.sub.wall=0.96 and R.sub.LED=0.5). For aperture fractions around
f.sub.opt, the quality parameter Q drops noticeably, i.e. the curve
flattens out, if the reflectivities R.sub.LED and R.sub.wall
decrease.
[0088] Furthermore, FIG. 13 shows calculations of the quality
parameter Q as a function of the aperture fraction f for various
values .theta..sub.LED at constant R.sub.wall=0.98 and
R.sub.LED=0.5 and for .theta..sub.c=90.degree.. As can be seen, the
quality parameter Q increases over the entire range of f with
increasing packing density .theta..sub.LED.
[0089] FIGS. 14, 15 and 16 show calculations for the light
concentration factor L(.theta..sub.c) at realistic reflectivities
R.sub.wall=0.98 and R.sub.LED=0.5 for various values of the
aperture fraction f (in FIG. 14 with a constant packing density
.theta..sub.LED=0.05) and for various values .theta..sub.LED (in
FIG. 15 with a constant aperture fraction f=0.05 and in FIG. 16
with a constant aperture fraction f=0.1). In all figures the lines
are drawn subject to the constraint according to Equation (7).
[0090] It is evident that, at least for .theta..sub.c=60.degree.
(general lighting applications), use of a light engine according to
the invention allows a significant brightness concentration to be
accomplished with a numerical value up to a factor 5 at 80% lumen
output (i.e. T=0.8). Also, higher light concentration factors L are
achievable by reducing the aperture fraction f but at the cost of a
reduced lumen efficiency.
[0091] Taking for granted that R.sub.wall and R.sub.LED are always
chosen as high as practically possible, it is primarily the packing
density .theta..sub.LED of the LED elements on the inner wall that
affects the performance as a function of the aperture fraction f. A
compromise will always have to be sought between brightness on the
one hand and lumen efficiency on the other hand. Also the total
required lumen output must be considered, whereby the size of the
light engine is directly proportional to the total lumen
output.
[0092] In case a high lumen efficiency is most important, it is
advisable to choose the aperture fraction f.apprxeq.0.10-0.12 at a
low .theta..sub.LED.apprxeq.0.01. This allows for T.apprxeq.0.8 and
B.apprxeq.0.07 which, at .theta..sub.c=90.degree., is still seven
times brighter than the screen-averaged brightness of the mounting
wall. The brightness concentration factor L(.theta..sub.c)
decreases at decreasing .theta..sub.c but remains substantial down
.theta..sub.c=40.degree..
[0093] In case a high brightness is most important, it is advisable
to choose a higher LED packing density
.theta..sub.LED.apprxeq.0.05, or even more if practicable. To raise
the maximum attainable LED packing density, cooling of the LED
elements should be provided, for example, by means of a matching
refractive index cooling liquid as proposed above. At
f.apprxeq.0.1, one has a smaller T=0.65 but a higher brightness
ratio B=0.3, which, at .theta..sub.c.apprxeq.=90.degree., is still
six times brighter than the screen-averaged brightness
B=.theta..sub.LED of the mounting wall. The brightness can be
further increased by decreasing the aperture fraction f down to,
for example, f=0.05. At f.apprxeq.0.1, the quality parameter Q
significantly improves at increasing .theta..sub.LED. To obtain
most benefit from the light engine according to the invention, it
is therefore of great interest to increase .theta..sub.LED up to
levels at and beyond .theta..sub.LED=0.10
[0094] Although the present invention has been disclosed in the
form of preferred embodiments and variations thereon, it will be
understood that numerous additional modifications and variations
could be made thereto without departing from the scope of the
invention. For the sake of clarity, it is also to be understood
that the use of "a" or "an" throughout this application does not
exclude a plurality, and "comprising" does not exclude other steps
or elements.
* * * * *